Using data that track 200 years of changes in the Chesapeake Bay, a team of environmental scientists has found that nutrient pollution has fundamentally altered the variety of aquatic life in the bay.
"Nature doesn't disappear ... it changes," said W. Michael Kemp, a researcher at the University of Maryland Center for Environmental Science's Horn Point Laboratory and the study's lead author.
In the Chesapeake, the study found, the influx of nitrogen and phosphorus has meant sharp declines in the number of bottom-dwelling organisms, such as oysters, blue crabs, sturgeon and flounder.
In contrast, less-desirable plants and animals that live in the now nutrient-rich upper layers of the bay's waters have thrived. These include algae, plankton and menhaden. Bacteria have also become a much more important component of the bay's ecosystem, the study found.
The study does not place blame for the shifts in fisheries production entirely on nutrient pollution, though, acknowledging the roles that climate and overfishing also play.
"The work pulls together in an integrated form more than 50 years of research," Kemp said. "We probably know more than we think we know. There's a lot to learn from digging into the data we've already collected."
Beth McGee, a senior scientist at the Chesapeake Bay Foundation who specializes in water quality, said the study was a comprehensive synthesis of work stretching back more than 20 years.
"It's what a lot of us were thinking all along," she said.
Using sediment samples, Kemp and the study's co-authors were able to trace increases in nutrient flow to the Chesapeake more than 200 years back. The most drastic changes occurred in the past 50 years, as nutrient runoff reduced the size of sea grass beds and lowered concentrations of dissolved oxygen. Excess nutrients can also cause the large algae blooms that lead to "dead zones" in the bay.
McGee said she was struck by the study's findings about the relationship between the amount of nitrogen coming into the bay and overall water quality. "The bay isn't as resilient as it used to be," she said.
The study, published in the Nov. 21 issue of Marine Ecology Progress Series, also discusses the relative effectiveness of various restoration efforts, including attempts to promote the health of sea grass beds, oyster reefs and tidal marshes, which filter nutrients out of the watershed.
